Vice President Mike Pence says the US will keep up pressure on Iran as the nuclear deal between the Islamic Republic and world powers appears to be crumbling.

Pence said in a speech on Monday in Washington that the US will “never allow Iran to obtain a nuclear weapon.”

Iran threatened on Monday to restart deactivated centrifuges and ramp up enrichment of uranium to 20 per cent purity as its next potential big moves away from a 2015 nuclear agreement that Washington abandoned last year.

The threats, made by Tehran’s nuclear agency spokesman, would go far beyond the small steps Iran has taken in the past week to nudge stocks of fissile material just beyond limits in the pact.

That could raise serious questions about whether the agreement, intended to block Iran from making a nuclear weapon, is still viable.

The two threats would reverse major achievements of the agreement, although Iran omitted important details about how far it might go to returning to the status quo before the pact, when Western experts believed it could build a bomb within months.

French President Emmanuel Macron was sending his top diplomatic adviser to Iran on Tuesday and Wednesday to try and help defuse tensions, a presidential official said.

In his prepared remarks, the vice president said that the US was willing to talk with Iran but “America will not back down.”

He did say that Iran should not “confuse American restraint with a lack of American resolve.”

“We hope for the best, but the United States of America and our military are prepared to protect our interests and protect our personnel and our citizens in the region,” Pence said.

Pence said Washington would continue to pressure Iran with sanctions and said President Donald Trump would “never allow Iran to obtain a nuclear weapon.”

Pence noted that Iran said it has begun enriching uranium beyond limits set by 2015 nuclear agreement.

He said it was part of the Islamic Republic’s “malign” activities that included recent attacks on oil tankers in the Arabian Gulf region.

Trump has put increasing pressure on Iran after withdrawing from what Pence called the “disastrous” agreement between Iran and world powers that imposed restrictions on its nuclear programme in exchange for sanctions relief.

“The United States does not seek a war with Iran. We are willing to talk. We are willing to listen. But America will not back down,” Pence had been set to say in his speech ahead of delivery.

In a separate standoff, Iran’s foreign minister accused Britain of “piracy” for seizing an Iranian oil tanker last week. Britain says the ship was bound for Syria in violation of European Union sanctions.

Britain, one of Washington’s main allies, was drawn deeper into the confrontation last week when its Royal Marines seized an Iranian tanker entering the Mediterranean off Gibraltar over separate sanctions against Syria.

“Iran is neither a member of the EU nor subject to any European oil embargo. Last I checked, EU was against extraterritoriality. UK’s unlawful seizure of a tanker with Iranian oil on behalf of #B_Team is piracy, pure and simple,” Iran’s Foreign Minister Mohammad Javad Zarif tweeted on Monday, using ‘B team’ as a derisory term for the Trump administration.

Iran says the deal allows it to respond to the US breach by reducing its compliance, and it will do so every 60 days.

Zarif also tweeted that world powers will not be able to negotiate a better deal than the 2015 nuclear deal.

“#B_Team sold @realDonaldTrump on the folly that killing #JCPOA thru #EconomicTerrorism can get him a better deal,” Zarif wrote, referring to the deal by its acronym for the Joint Comprehensive Plan of Action.

“As it becomes increasingly clear that there won’t be a better deal, they’re bizarrely urging Iran’s full compliance. There’s a way out, but not with #B_Team in charge.”

Agencies